Comment by cubefox

A reusable lower stage with powered landing also had never been flown to orbit at that time. And in contrast to aerospike engines, which had been tested before on the ground [1], you can't do ground testing with rocket stage landings.

I think SpaceX didn't try to develop aerospike nozzles because the advantages probably aren't that large compared to the mixed nozzle design they are currently using. They also reused the same ceramic heat shield material developed for the space shuttle instead of developing something new.

Compare that to the cancelled "VentureStar": It would have used both linear aerospike engines and a new metallic thermal protection system (TPS) instead of a ceramic one. I remember an interview where Musk answered the question of why they aren't doing aerospikes or metallic heat shields etc, that there are many ways to skin a cat. They are only doing one thing that they think will work, which is not necessarily the best possible solution, but potentially faster or cheaper to develop.

[1] https://youtube.com/watch?v=FcW9kUUTfxY